Study of elevated- and ground pool fire flame horizontal lengths in cross airflows: Air entrainment change due to Coandă effect

Abstract

The present study investigates experimentally the flame horizontal lengths of elevated pool fires and compares with those of ground pool fires in cross airflows. This, as a fundamental problem considering air entrainment and combustion structure change due to Coandă effect resulted from ground at leeward side, has not been quantified but is of practical significance. Experiments were performed employing both elevated- and ground square propane pool fires of various heat release rates in cross airflows from 0∼3.0 m/s with 224 test conditions in all. The overall flame horizontal length could be physically divided into two parts: 1) cross flow momentum-dominated length (base-drag part, Lm); 2) buoyancy-dominated length (plume-like part, Lb). Both the Lm and the overall flame horizontal length showed to be significantly larger for a ground pool fire than those of an elevated one, due to the ground constraint effect on air entrainment at leeward side (Coandă effect). Lb/(u2/g) or Lm/(u2/g) of elevated fires is proportional to that of ground fires. Large eddy simulations for two representative fire scenarios were further conducted utilizing an in-house version of FireFOAM. The purpose of the simulations was to uncover the underlying physics of combustion and flow structures observed in the experiment. Finally, a model was proposed based on analysis of air entrainment, momentum- and buoyancy effects, which described well the overall flame horizontal lengths of both elevated- and ground pool fires generally as a function of non-dimensional heat release rate, normalized volumetric air entrainment and Froude numbers using Lm and Lb as length scales. This work provides new observation and basic quantification on elevated- and ground pool fire flame horizontal lengths considering air entrainment change due to Coandă effect in wind.